tail head cat sleep
QR code linking to this page

Manual Pages  — IICBUS


iicbus – I2C bus system



device iicbus device iicbb

device iic device ic device iicsmb


The iicbus system provides a uniform, modular and architecture-independent system for the implementation of drivers to control various I2C devices and to utilize different I2C controllers.


I2C is an acronym for Inter Integrated Circuit bus. The I2C bus was developed in the early 1980's by Philips semiconductors. Its purpose was to provide an easy way to connect a CPU to peripheral chips in a TV-set.

The BUS physically consists of 2 active wires and a ground connection. The active wires, SDA and SCL, are both bidirectional. Where SDA is the Serial DAta line and SCL is the Serial CLock line.

Every component hooked up to the bus has its own unique address whether it is a CPU, LCD driver, memory, or complex function chip. Each of these chips can act as a receiver and/or transmitter depending on its functionality. Obviously an LCD driver is only a receiver, while a memory or I/O chip can both be transmitter and receiver. Furthermore there may be one or more BUS MASTERs.

The BUS MASTER is the chip issuing the commands on the BUS. In the I2C protocol specification it is stated that the IC that initiates a data transfer on the bus is considered the BUS MASTER. At that time all the others are regarded to as the BUS SLAVEs. As mentioned before, the IC bus is a Multi-MASTER BUS. This means that more than one IC capable of initiating data transfer can be connected to it.


Some I2C device drivers are available:

Devices Description
iic general i/o operation
ic network IP interface
iicsmb I2C to SMB software bridge


The I2C protocol may be implemented by hardware or software. Software interfaces rely on very simple hardware, usually two lines twiddled by 2 registers. Hardware interfaces are more intelligent and receive 8-bit characters they write to the bus according to the I2C protocol.

I2C interfaces may act on the bus as slave devices, allowing spontaneous bidirectional communications, thanks to the multi-master capabilities of the I2C protocol.

Some I2C interfaces are available:

Interface Description
pcf Philips PCF8584 master/slave interface
iicbb generic bit-banging master-only driver
lpbb parallel port specific bit-banging interface
bktr Brooktree848 video chipset, hardware and software master-only interface


The operating frequency of an I2C bus may be fixed or configurable. The bus may be used as part of some larger standard interface, and that interface specification may require a fixed frequency. The driver for that hardware would not honor an attempt to configure a different speed. A general purpose I2C bus, such as those found in many embedded systems, will often support multiple bus frequencies.

When a system supports multiple I2C buses, a different frequency can be configured for each bus by number, represented by the %d in the variable names below. Buses can be configured using any combination of device hints, Flattened Device Tree (FDT) data, tunables set via loader(8), or at runtime using sysctl(8). When configuration is supplied using more than one method, FDT and hint data will be overridden by a tunable, which can be overridden by sysctl(8).

Device Hints

Set hint.iicbus.%d.frequency to the frequency in Hz, on systems that use device hints to configure I2C devices. The hint is also honored by systems that use FDT data if no frequency is configured using FDT.

Flattened Device Tree Data

Configure the I2C bus speed using the FDT standard clock-frequency property of the node describing the I2C controller hardware.

Sysctl and Tunable

Set dev.iicbus.%d.frequency in loader.conf(5). The same variable can be changed at any time with sysctl(8). Reset the bus using i2c(8) or the iic(4) I2CRSTCARD ioctl to make the change take effect.


bktr(4), fdt(4), iic(4), iicbb(4), lpbb(4), pcf(4), i2c(8)


The iicbus manual page first appeared in FreeBSD 3.0 .


This manual page was written by Nicolas Souchu.

IICBUS (4) January 15, 2017

tail head cat sleep
QR code linking to this page

Please direct any comments about this manual page service to Ben Bullock. Privacy policy.

Not only is UNIX dead, it's starting to smell really bad.
— Rob Pike